1. Field of the Invention
This invention relates to methods for producing optical waveguides, and other such components or devices which require patterns of altered index of refraction or thickness in transparent polymer multilayer structures. In particular this invention relates to methods for producing such optical waveguides, and other such components or devices upon irradiation with actinic radiation and subsequent treatment. These optical waveguides, and other such components or devices may be used in fields ranging, for example, from telecommunications, to optical computing, data storage, displays, and sensors.
2. Description of Related Art
Current technology for manufacturing polymer optical waveguides includes techniques well known in the art, such as, for example, reactive ion etching (RIE) or laser ablation of the core or cladding layers to provide rib waveguides, ion implantation, or photobleaching of the core or cladding layers. In addition, optical waveguides can be formed by photodefinition, a process in which a layer of organic material, for example a polymer, is deposited on another layer of material, and predetermined regions of the polymer layer are subjected to actinic radiation in order to alter the refractive index of these regions.
The first step in the photodefinition process is the deposition of one or more transparent photosensitive monomers onto a substrate. The chosen monomers are selected on the basis of their ability to dimerize or polymerize upon exposure to light and their relative indices of refraction. When such monomers are subsequently irradiated with a pattern of light, for example ultraviolet (UV) light, polymerization is initiated in the exposed material. Since the polymer is formed from reacted monomers, monomer concentration is depleted in the irradiation region, creating a gradient in chemical potential between the exposed and the unexposed areas. The resulting gradients are unstable, and the smaller-molecular-weight monomers surrounding the photochemically reacted material will diffuse as much as possible into the polymerized regions, increasing the index and creating a feature in the planar layer. At least a single, subsequent exposure of the entire material system to high-intensity UV or thermal radiation, for example, may be used to complete the process. This fixing step polymerizes all remaining reactants, creating a system that is effectively fully cured and inert. An important aspect of this procedure is that the resulting physical density in the patterned (exposed) regions is either higher or lower than in the unpatterned material. This density difference causes an associated difference in index of refraction. Importantly for photonic devices, this index change is stable against further processing and pronounced enough to enable efficient waveguiding. A process of creating optical waveguides by photoinduced diffusion is described by Chandross et al. (U.S. Pat. Nos. 3,809,732; 3,809,696; 3,993,485, and; 3,953,620, all of which are incorporated herein by reference) and has been used extensively to create photonic devices.
It should be noted that photo-imaging might also be considered to be a form of photodefinition. In this process, an imaging layer is deposited on a substrate, and an image transferred into the surface of or throughout the entire imaging layer. The imaging layer in this instance would contain the photosensitive molecules. This technique would enable, for example, a waveguide to be fabricated by coating a substrate/lower cladding layer with photoresist or electron beam sensitive material such as PMMA, impinging light from a suitable light source (e.g., light in an ultraviolet region) upon a photomask having a predetermined pattern to transfer the pattern to the photoresist, and developing the photoresist to provide photoresist having the predetermined pattern. In this case, the predetermined pattern is the pattern for the rib/core of the waveguide, and material from the imaging layer has been removed to leave the free-standing feature. The waveguide fabrication is completed by depositing additional layer(s) over the patterned rib/core layer, these layers being additional core or cladding layers. It will be appreciated, that the optical feature, or rib, described here, is not dependent upon diffusion for its definition.
To applicants"" knowledge, the prior art of photodefinition of polymer optical waveguides requires that the photodefinition process occur effectively immediately following the deposition of the photodefinable layer, and before deposition of the upper cladding layer, unless this is to be the photodefined layer. Consequently the photodefinition process necessitates that the layer coating/deposition process be interrupted and intermediate photodefinition processes or other processing and treatment effected before coating is finally resumed to complete manufacture of the device. Interruption of the coating process to perform photodefinition may ultimately lead to higher loss waveguides due to dust or dirt being deposited either at, or in close proximity to, the optical layer during processing, and a greater chance of the generation of defects in or near the optical layer due to the number of photodefinition processing steps that must be performed directly thereon.
FIGS. 1a to 1d illustrates schematically the technique by which photodefinition is conventionally practiced in the prior art. As shown in FIG. 1 a, the lower cladding 102 is deposited on the substrate 103 and cured, and a core layer 104 is then deposited over the lower cladding 102. The core layer 104 is then optically patterned, for example by photodefinition, as illustrated in FIG. 1c. In this example the optical patterning is lithographic, in which radiation 105 projected through a mask 106 is used to pattern the optical elements. Alternatively the exposure can result in a chemical change followed by removal of residual polymer/monomer using a wet etch, laser ablation, or further processing, to leave a rib waveguide (not shown). This step produces an area of elevated or reduced refractive index, 108, FIG. 1c. If necessary, the core layer is then locked or stabilized using suitable thermal or radiation exposure processing such that further refractive index change is unattainable. An upper cladding layer 110 is then deposited and cured, as indicted by FIG. 1d. It will be apparent that in this process, the layer 104 to be patterned, is processed to achieve photodefinition before subsequent layer(s) 110 are applied. This interruption in the layer coating process may ultimately lead to higher loss waveguides due to dust or dirt being deposited either at, or in close proximity to, the core/upper cladding interface during the photodefinition processing steps, or in processing, planarization or uniformity problems. It should be noted that the terms upper and lower are used herein solely for convenience in referring to specific layers. The layers they refer to are not intended to change if the structure is turned upside down or tilted.
The present invention provides optical waveguides, components or devices that may be defined after a complete optical multilayer structure/stack has been deposited. In addition, the utilization of multiple photosensitive molecules enables the required change in index for waveguide fabrication and increased ease of processing (e.g., fewer processing steps, better adhesion). Furthermore, the invention allows for a continuous or progressive fabrication process to be considered, enabling the reduction, if not the elimination, of the need to interrupt the coating process to perform waveguide photodefinition and/or not requiring the interruption of a continuous (roll) manufacturing multilayer structure/stack deposition process. Utilization of the current invention enables several advantages to be realized. These advantages include, for example, the following: lower loss waveguides due to less dust/dirt being deposited in, or in close proximity to, the optical layer during processing; the option for the deposition/coating processing to be performed independently from waveguide photo-patterning and the potential for increased ease of overall processing including a decreased number of fabrication steps and improved interlayer adhesion as a result of the uninterrupted layer coating process. This invention also has the flexibility to be used to position and create channel waveguides exactly where required within a multilayer structure to interconnect components.
According to the invention, roughly described, an optical structure is fabricated by forming an active layer including a photodefinable material on a substrate or on another underlying layer, forming an upper layer above the active layer, and then patterning the active layer by selective application of radiation through the upper layer. The upper layer is substantially transparent to radiation of the type required to activate the photodefinable material in the active layer.